Exemplary embodiments of the present invention relate to a semiconductor device, and more particularly to a vertical transistor having a buried junction by hybrid doping and a method for manufacturing the same.
With increasing integration of semiconductor devices, emphasis has been made to integrate a larger number of unit devices such as transistors in a limited substrate area. A MOSFET device having a planar structure and a planar-type transistor include junctions positioned on either side of a transistor gate on the surface of a substrate, thereby forming a horizontal channel. In order to integrate a greater number of transistors in a limited area of the substrate, a reduction in the channel length of each transistor is required.
However, reducing transistor channel length in a horizontal direction has met with many difficulties due to opposing physical limitations such as leakage current, short channel effect, and on-current reduction. The ability to further miniaturize planar structure in horizontal directions is approaching its limit. Accordingly, a vertical transistor structure has been proposed. This structure utilizes an active region that protrudes vertically as opposed to horizontally in the case of a planar structure.
In the vertical transistor structure, junctions should be formed over and under the channel set in the vertical direction. However, there are technical difficulties in forming a buried junction under the channel due to a geometrical factor associated with the vertical structure of the transistor. The distance between the buried junction and a channel region overlapping a vertical gate may vary depending on the formation position of the buried junction in the vertical direction and the diffusion depth of the junction. Such variation in distance may have an effect on a threshold voltage Vt of the channel. When the diffusion depth of the junction is insufficient, the distance between the channel and the buried junction may increase, thereby increasing the threshold voltage Vt of the channel. Therefore, the position of the junction with respect to the channel area and the diffusion profile of the junction need to be controlled with precision. Furthermore, when the dopant concentration of the junction is low, contact resistance of interconnections contacting the junction may increase.